A three-way catalyst which performs purification of the exhaust gases from a vehicle engine oxidizes carbon monoxide (CO) and hydrocarbons (HC), and reduces nitrogen oxides (NOx).
The oxygen used to oxidize CO and HC comes from a small amount of excess oxygen contained in combustion gases, and oxygen produced by reduction of NOx.
In a vehicle engine, so-called fuel cut is performed where the fuel supply to the engine is cut off under predetermined deceleration conditions. When fuel cut is performed, intake air is discharged in the exhaust gases and is not burnt. As a result, oxygen which sharply increases in the exhaust gases is adsorbed by the three-way catalyst, and an excess oxygen state continues in the three-way catalyst for some time after fuel cut is terminated. In this excess oxygen state, reduction of NOx is not effective, and the amount of NOx discharged increases.
Tokkai Hei 8-193537 published by the Japanese Patent Office in 1996 discloses a technique wherein the air-fuel ratio is made richer than the stoichiometric air-fuel ratio for a very short time immediately after fuel cut is terminated. In this prior art technique, the intake air amount during fuel cut or the oxygen amount adsorbed by the three-way catalyst from the time of fuel cut is calculated, and a richness degree is determined based on the adsorbed oxygen amount.
When the driver steps on the accelerator pedal, fuel cut is terminated. Fuel cut is terminated also when the engine rotation speed has become lower than a predetermined value even if the driver does not step on the accelerator pedal.
After a long fuel cut, the fuel injection amount must be increased by at least 10% to suppress increase of the NOx discharge amount immediately after fuel cut is terminated. However, if such an increase of fuel injection amount occurs when the driver does not step on the accelerator pedal, it will result in an increase of engine output torque not intended by the driver.